Discard candidates with an empty data type (#19172)

Fixes
tailwindlabs/tailwindcss-intellisense#1479

Maybe should close
tailwindlabs/tailwindcss-intellisense#1480 —
perhaps we can find a workaround there for older versions?

We're building up a class name in code to validate if something is a
valid variant: `{variant}:[color:red]`

if `{variant}` got replaced with `bg-[` then we'd produce
`bg-[:[color:red]` and this parsed as a valid candidate:
```
bg-[:[color:red]
^^                root: `bg`
   ^              data type: `` (empty string) — this should be invalid
     ^^^^^^^^^^   value: `[color:red`
```

The value isn't valid _but_ the syntax for arbitrary values is pretty
lax in core. Oxide already won't pick something like this up though so
no problem there. Only a problem for something like IntelliSense or
clients using the compile() API directly.

Author: thecrypticace

CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - _Experimental_: Add `@container-size` utility ([#18901](https://github.com/tailwindlabs/tailwindcss/pull/18901))
 
+### Fixed
+
+- Discard candidates with an empty data type ([#19172](https://github.com/tailwindlabs/tailwindcss/pull/19172))
+
 ## [4.1.15] - 2025-10-20
 
 ### Fixed

packages/tailwindcss/src/candidate.test.ts

@@ -1776,6 +1776,14 @@ it('should not parse compound group with a non-compoundable variant', () => {
   expect(run('group-*:flex', { utilities, variants })).toMatchInlineSnapshot(`[]`)
 })
 
+it('empty data types are invalid', () => {
+  let utilities = new Utilities()
+  utilities.functional('bg', () => [])
+  let variants = new Variants()
+
+  expect(run('bg-[:foo]', { utilities, variants })).toMatchInlineSnapshot(`[]`)
+})
+
 it('should parse a variant containing an arbitrary string with unbalanced parens, brackets, curlies and other quotes', () => {
   let utilities = new Utilities()
   utilities.static('flex', () => [])

packages/tailwindcss/src/candidate.ts

@@ -554,7 +554,7 @@ export function* parseCandidate(input: string, designSystem: DesignSystem): Iter
         if (!isValidArbitrary(arbitraryValue)) continue
 
         // Extract an explicit typehint if present, e.g. `bg-[color:var(--my-var)])`
-        let typehint = ''
+        let typehint: string | null = null
         for (let i = 0; i < arbitraryValue.length; i++) {
           let code = arbitraryValue.charCodeAt(i)
 
@@ -580,6 +580,8 @@ export function* parseCandidate(input: string, designSystem: DesignSystem): Iter
           continue
         }
 
+        if (typehint === '') continue
+
         candidate.value = {
           kind: 'arbitrary',
           dataType: typehint || null,